As described in, for example, Japanese Laid-Open Patent Application No. 2000-184134 (published on Jun. 30, 2000) and Japanese Laid-Open Patent Application No. 2003-264671 (published on Sep. 19, 2003), there have been image reading apparatuses that can read image information from the both sides of a document, while transporting the document. FIG. 1 is a schematic view of such a conventional image reading apparatus (scanner B) that can read out images from the both sides. Referring to FIG. 1, the outline of the conventional image reading apparatus that can read out images from the both sides will be described.
As shown in the figure, the scanner B of the conventional art includes an ADF device α1 and a front-side optical section α2. The ADF device α1 includes a document tray 1 and a paper output tray 2. Between the document tray 1 and the paper output tray 2, a predetermined transportation path 3 is provided for transporting a document to the paper output tray 2. Along the transportation path 3, transportation rollers 4 (an example of a transportation section) are formed at different positions along the transportation path 3. As the transportation rollers 4 are driven so as to rotate, the document from which an image is read out is transported along the transportation path 3.
In the front-side optical section α2, a first image reading section X1 is formed for reading an image from the front side (one side) of the document. The first image reading section X1 reads out the image, when the document which is transported along the transportation path 3 passes through a predetermined first readout position 5.
The first image reading section X1 includes members such as an exposure device 6, an optical guiding mirror 7, a lens 8, and a CCD 9. When the document passes through the first readout position 5, the exposure device 6 emits image reading light onto the document. The image reading light is reflected on the document, and is guided to the lens 8 by the optical guiding mirror 7. The image reading light provides an image on the CCD 9, on account of the lens 8. The image reading light is converted into an electric signal by the CCD 9, and supplied to an image processing control section (see FIG. 4, described later). As a result, the image information on the front side of the document is read out by the first image reading section.
In the meantime, the ADF device α1 is provided with a second image reading section X2 that reads out an image from the back side (the other side) of the document. The second image reading section X2 reads out the image from the back side of the document, when the document which is transported along the transportation path 3 passes through a second readout position 10 which is downstream of the first readout position 5, in the document transport direction. Being similar to the first image reading section X1, the second image reading section X2 includes members such as an exposure device 6′, an optical guiding mirror 7′, a lens 8′, and a CCD 9′.
On the CCD 9′, an image is provided by image reading light which irradiates the back side of the document. The image reading light is converted into an electric signal by the CCD 9′, and is supplied to the image processing control section. As a result, the image information on the back side of the document is read out by the second image reading section X2. On the transportation path 3, a bent portion β1 that protrudes upward and a bent portion β2 that protrudes downward are formed between the first readout position 5 and the second readout position 10.
The first readout position 5 and the second readout position 10 cannot be overlapped with one another on the transportation path 3. This is because, if these positions are provided at the same location, the image reading light for reading out an image from one side of the document enters the image reading section that reads out an image from the other side, and hence the image reading light becomes inaccurate.
As shown in FIG. 1, the transportation rollers 4 (an example of the transportation section) are provided at different positions, including a position upstream of the first readout position 5 in terms of the transportation direction, and a position downstream of the second readout position 10 in terms of the transportation direction. The transportation rollers 4 serially transport the documents along the transportation path 3. While the document is transported, the rotation speed of each transportation roller 4 is controlled based on the transportation speed of the document, which is set by the image processing control section. (It is noted that the transportation speed is worked out by multiplying the rotation speed by the diameter of the roller.) The lower the position of the transportation roller 4 in terms of the transportation direction is, the higher the transportation speed of the transportation roller 4 is. This prevents the document from being warped while being transported.
Therefore, the speed of the document passing through the first readout position 5 is lower than the speed of the document passing through the second readout position 10.
The reading cycle (reading speed) of the CCD 9 of the first image reading section X1 and the reading cycle (reading speed) of the CCD 9′ of the second image reading section X2 are set in accordance with the speed of the document at the time of passing through the first readout position 5 and the speed of the document at the time of passing through the second readout position 10, respectively. For example, in a case where the ratio between the speed of the document passing through the first readout position 5 and the speed of the document passing through the second readout position 10 is 1:1.05, the ratio between the reading cycle of the CCD 9 and the reading cycle of the CCD 9′ is also set at 1:1.05.
In this manner, the reading cycles of the CCDs 9 and 9′ are set in accordance with the passing speeds at the respective readout positions. In other words, the ratio between the reading cycle of the CCD 9 and the reading cycle of the CCD 9′ is set so as to be identical with the ratio between the passing speed at the first readout position 5 and the passing speed at the second readout position 10. This causes the magnification of the image read out from the front side and the magnification of the image read out from the back side to be identical with each other. In other words, the arrangement above prevents a stored image, which has been read out from one side, from being larger or smaller in size as compared to a stored image read out from the other side.
However, for example, as the transportation roller 4 is worn down over time, the diameter of the roller changes. If the diameter of the roller changes, the ratio between the passing speeds at the respective readout positions also changes even if the settings of the transportation speeds are not varied. As a result, the ratio between the passing speeds does not correspond to the ratio between the reading cycles of the respective CCDs that read the image information from the front side and the back side of the document. On this account, the magnification of the image read out from the front side becomes different from the magnification of the image read out from the back side.